Table 3.
Multivariable Cox regression analyses of the retrospective cohort study sample
| No. Exposed | No. of ILD Cases | Total Person-Years | ILD Event Rate* | Adjusted HR† | 95% CI | P Value | ||
|---|---|---|---|---|---|---|---|---|
| RF/ACPA seropositivity | RF−/ACPA− | 335 | 12 | 2,107 | 5.7 | Ref. | — | — |
| RF+/ACPA− or RF−/ACPA+ | 374 | 13 | 2,313 | 5.6 | 1.12 | 0.50–2.52 | 0.78 | |
| RF+/ACPA+ | 1,519 | 58 | 9,861 | 5.9 | 1.05 | 0.54–2.02 | 0.90 | |
| RF titer analyses | ||||||||
| RF titer category | Negative | 527 | 18 | 3,243 | 5.6 | Ref. | — | — |
| Low positive (15–45 IU/ml) | 346 | 9 | 2,277 | 4.0 | 0.77 | 0.34–1.74 | 0.53 | |
| High positive (>45 IU/ml) | 1,355 | 56 | 8,761 | 6.4 | 1.08 | 0.62–1.89 | 0.79 | |
| RF titer quartiles | 1 | 557 | 19 | 3,395 | 5.6 | Ref. | — | — |
| 2 | 557 | 13 | 3,638 | 3.6 | 0.70 | 0.34–1.44 | 0.34 | |
| 3 | 557 | 20 | 3,670 | 5.4 | 1.01 | 0.53–1.92 | 0.99 | |
| 4 | 557 | 31 | 3,578 | 8.7 | 1.43 | 0.77–2.65 | 0.25 | |
| Log-transformed RF titer | Per 1 log-transformed unit | — | — | — | — | 1.15 | 1.00–1.32 | 0.05 |
| Log-transformed RF titer with spline term | Per 1 log-transformed unit | — | — | — | — | 0.85 | 0.62–1.17 | 0.31 |
| Spline term at 4.5 | — | — | — | — | 1.68 | 1.02–2.77 | 0.04 | |
| ACPA titer analyses | ||||||||
| ACPA titer category‡ | Negative | 517 | 19 | 3,284 | 5.8 | Ref. | — | — |
| Low positive (5–15 U/ml) | 93 | 0 | 497 | 0.0 | — | — | — | |
| High positive (>15 U/ml) | 1,618 | 64 | 10,500 | 6.1 | 1.05 | 0.62–1.79 | 0.85 | |
| ACPA titer quartiles | 1 | 557 | 19 | 3,475 | 5.5 | Ref. | — | — |
| 2 | 557 | 20 | 3,458 | 5.8 | 1.04 | 0.54–2.02 | 0.90 | |
| 3 | 557 | 15 | 3,704 | 4.0 | 0.78 | 0.39–1.56 | 0.48 | |
| 4 | 557 | 29 | 3,644 | 8.0 | 1.45 | 0.80–2.65 | 0.22 | |
| Log-transformed ACPA titer | Per 1 log-transformed unit | — | — | — | — | 1.04 | 0.94–1.15 | 0.50 |
Definition of abbreviations: ACPA = anti–citrullinated protein antibody; CI = confidence interval; DAS28 = Disease Activity Score in 28 Joints; HR = hazard ratio; ILD = interstitial lung disease; No. = number; RA = rheumatoid arthritis; RF = rheumatoid factor; Ref. = reference.
*
Event rate is per 1,000 person-years.
†
Multivariable models are adjusted for age at enrollment, sex, race (white, African American, or other), smoking history, RA disease duration, and baseline articular disease severity (DAS28).
‡
No subjects with incident ILD had a low-positive ACPA titer, whereas 19 had a negative titer and 64 had a high-positive titer.